Lifespan and circadian rhythms will be investigated in mice subjected to repeated shifting of synchronizers (daily lighting and/or feeding schedules). Possible contributory factors to be examined are: 1) heredity; 2) age at first exposure; 3) interaction of synchronizers; 4) frequency of shifting; 5) extent of shift; and 6) direction of shift. These studies on a well-controlled model relate to possible adverse or beneficial effects of shift-work in man.